Every calculator on this site is built from published industry conventions and standard material yields rather than proprietary formulas. This page documents what those are, so you can check our work instead of taking it on faith.
The core principle
Nearly every material estimate reduces to the same three steps: measure an area or volume, divide by the coverage or yield of the material, then add an allowance for waste. Where our numbers differ from another calculator's, the difference is almost always in the third step — how much waste is assumed — not in the geometry.
We state the waste allowance on every page and, wherever practical, let you change it. A calculator that hides its waste factor is hiding the only number that is genuinely a judgment call.
Standards and conventions we follow
These are the published references our figures are consistent with. We are not affiliated with any of these bodies, and we link to them so you can verify a figure against the primary source.
| Topic | Convention we follow | Reference body |
|---|---|---|
| Concrete mix, strength, curing | PSI classes, 28-day design strength, 7-day moist cure | American Concrete Institute (ACI) |
| Control joint spacing | Spacing in feet ≤ 2–3× slab thickness in inches | ACI |
| Tile substrate and mortar beds | Bonded/unbonded mortar bed thickness, deflection limits | Tile Council of North America (TCNA) |
| Drywall levels of finish | Levels 0–5 as defined in the joint industry standard | Gypsum Association |
| Lumber nominal vs actual sizes | Dressed dimensions per the softwood lumber standard | American Lumber Standard Committee |
| Accessible routes, slopes, widths | 36 in clear width, 2% max cross slope, 5% ramp threshold | US Access Board (ADA) |
| Roof pitch, ventilation, underlayment | Slope factors, 1:150 / 1:300 ventilation ratios | International Residential Code (IRC) |
| Stucco systems and thickness | 7/8 in three-coat over lath; 1/2 in two-coat over masonry | IRC / ASTM C926 |
Material yields we assume
Bag yields are the manufacturer-published figures that are consistent across all major North American brands. Where a product genuinely varies between manufacturers we say so on the page and tell you to check the bag.
| Material | Assumed yield |
|---|---|
| Concrete mix, 40 / 60 / 80 lb bag | 0.30 / 0.45 / 0.60 cu ft |
| Portland cement, 94 lb bag | 1.0 cu ft |
| Mason/play sand, 50 lb bag | ~0.5 cu ft |
| Sand/topping mix, 60 lb bag | 0.5 cu ft |
| Mulch bag | 2 or 3 cu ft as sold |
| Gravel (bulk) | ~1.4 tons per cubic yard |
| Joint compound, ready-mixed | ~1 gallon per 100 sq ft at Level 4 |
| Paint | 350–400 sq ft per gallon, one coat, smooth surface |
| Asphalt shingles | 3 bundles per 100 sq ft square |
| Mortar bed (deck mud) | Finished volume ≈ loose sand volume |
Waste allowances
Waste is the number most often argued about, so here is exactly what we assume by default:
- 10% — concrete, paint, straightforward tile and flooring layouts, framing lumber
- 15% — diagonal tile layouts, decking, trim, and anything with many angled cuts
- 15–20% — cut-up roofs with multiple hips, valleys, and dormers
- 20–30% — hardwood you are cutting around defects for, and wallpaper with a large pattern repeat
- 5–10% — sod, more where beds are curved
Rounding
We round up to whole purchasable units — bags, sheets, rolls, bundles, pallets — because you cannot buy two-thirds of a bag. Volumes and areas are shown to one or two decimals so you can see the underlying figure before it is rounded. This means our bag counts are sometimes one higher than a calculator that rounds to the nearest unit. Running one bag short mid-pour is far more expensive than one spare bag.
What these calculators cannot do
These are planning estimates, not engineering. They tell you roughly how much material to buy. They do not size structural members, verify load paths, confirm code compliance in your jurisdiction, or replace a permit, an inspection, or a licensed professional. For anything structural, or anything an inspector will sign off on, have it designed and checked by a qualified engineer or contractor.
Specific limits worth knowing:
- Local codes override us. Frost depth, minimum footing sizes, permit thresholds, and setback rules are all local. We give common North American values; your jurisdiction decides.
- Real sites are not flat. Subgrade variation routinely pushes real concrete usage above the calculated figure — that is what the waste allowance is for, but a badly graded base can beat it.
- Costs move. Any prices shown are broad planning ranges, not quotes, and they vary sharply by region and season.
- Manufacturer figures win. If the bag in your hand states a different yield than we assume, use the bag.
Corrections
If you find a figure you believe is wrong, we want to know — a calculator is only worth using if it is right. See the about page for how to get in touch, and please include the page and the number you are questioning.